mode optical waveguide branches 13 and 14, a multi
mode waveguide 19 is formed. The multi mode optical ^ _ lit ^ (4)
waveguide 19 diverges at 20 into two optical wave- * guides 21 and 22 having different dimensions.
In operation of the embodiment illustrated in FIG. 1, 5 Those skilled and knowledgeable in the pertinent arts when no potential is applied to the electrodes 15, 16, wil1 appreciate that the concept of the present invention and 17, light in the single mode waveguide 11 propa- does not require that equal and opposite electric fields gates with a mode velocity and is divided equally at be applied to the two optical waveguide branches 13 12 into the two optical waveguide branches 13 and 14. and 14 t0 produce the desired phase difference but apWhen the light energy thus propagated reaches the '0 proximately twice the electric potential could be appoint 18, it has travelled the same distance in each opti- Plled t0 the Pair of electrodes 15 and 16 or the other pair cal waveguide branch since they are substantially of 16 and 1710 produce substantially the same phase differidentical optical length and therefore the reconverging ence a« is given effect by generating equal and opposite light is in phase and combines constructively, produc- electric fields. However, it will be equally well appreciing the lowest order mode which propagates with ve- 15 ated that in practice it is generally desirable to employ locity 0,' in the optical waveguide section 19. minimum operational electric potentials; thus, the equal
The light energy propagates in the optical waveguide and °PPoslte electnc fleld technique is generally pre
section 19 to the point 20 where it will continue to . , .
propagate in either the optical waveguide section 21 or u lf?h? aPPhed el,ectncal P°tenhal » su<* tha' = * the optical waveguide section 22 depending on which 20 *e hfyn optical wavegu,de branches 13 and H will of the mode propagation constants in the two optical be outA°J Phase and J"3 des ruct.vely mterfere. Stated •j V.■ .,, „ t , , „ . in a different way, the application of an appropriate
waveguide sections 21 or 22 most closely matches the , . • , . „• , ri ,• . t 5 •
° , , .. . . o , electrical potential causes the light propagating in the
propagation and velocity constant p,. .. . \. ,„ . - T j j
° , i ^ -i * \.- t\r- r J * optical section 19 to shift from the lower order mode
However, when an electrical potential V is applied to ,. v. .. * * o < * Jj A
,w ,m i_ v ui J 25 with propagation constant/3i to the second order mode
electrodes 15, 16, and 17 through a suitable ganged , , . K ^ 0 „ , .. : ... t
,' ... ° J ■ (which has a null at its center) with propagation con
switch means schematically represented in an open „ >
condition in FIG. la and in closed condition in FIGS. ^ and u ^ mode files a§ a
2a and 2b such that the polarity of electrode 16 is oppo- ... of ... ^ the ica, hs of the em. site the polarity of electrodes 15 and 17, a change is JQ of FIG. la when it is operated ^ a switch.
caused in the optical properties of the optical wave- Such operation results from the choice Gf dimensions of guide branches 13 and 14 depending upon the electro the Qptical waveguide section 21 and 22 to be such that optical responses of the type of material employed, its the propagation constant, 0,', of the lowest mode in orientation, and the magnitude of the applied electnc optical waveguide section 21 matches the lowest mode field as well. 35 m optical waveguide portion 19, i.e., /3,' = /3,".
In typical operation, the applied electrical potential V pr0pagation constant of the lowest mode of
will produce an electric field E between electrodes 15 waveguide 22 is chosen to be the same as the second and 16 which is opposite in sense to the electric field mo^e 0f ^ waveguide section 19, i.e., /32' = 0,'". developed between electrodes 16 and 17. In the optical it should be noted that the embodiment of the present waveguide branch 13 the applied electric field produces 40 invention illustrated in FIG. 1 may be operated as an a small change of refractive index for TE (polarized amplitude modulator. In such operation the waveguide parallel to the plane of the device) modes which may be sections 21 and 22 are not required, and the optical expressed as waveguide portion 19 need not only be single mode in
„3 (1) 45 As indicated schematically in FIGS. 2a and 2b, the
A»re = — este application of a voltage to the electrodes of the embodi
ment of the present invention illustrated in FIG. la _ «i E STM causes variation in the effective optical length of the
2 two optical waveguide branches so that interferometer
» JC1 „ . ._ .50 type performance is realized by utilization of the electro
where 5reand S^are found from he specific type and jc rties of the tical waveguide material. Ac
onentation of the crystal. In general 6re^SrMexcept in cordinglv> the optical waveguide interferometer certain orientations. modulator-switch of the present invention may be con
For simplicity and clarification of explanation it may sidered t0 ^ a mode converter) idealiy with substanbe assumed that 8re = 8TE = 8. The phase shift induced 5J tially equal optical length of the two optical waveguide in the light by this small change of refractive index may branches 13 and 14 of FIG. la, the light propagating be expressed as along the two optical guide branches being recombined
as shown in FIG. 2a to produce the lowest ord^r mode. _ 2ir £m W However, upon the application of an electric field so
* 2 60 as to cause a phase shift or n-radians between the two
optical waveguide branches 13 and 14, the recombinawhere X is the wavelength of the light and / is the length tion of the propagated light results in a light field distriof the waveguide over which the electric field is ap- bution having a value of zero at the center of the recompiled, bined light energy, i.e., the second order mode as shown In the optical waveguide branches 13 and 14 the 65 in FIG. 2b. Accordingly, the present invention will phase shift is equal but opposite in sense so that the operate as an amplitude modulator requiring no external phase difference between light propagated along the polarizers with the only requirement being that the exit two optical waveguide branches 13 and 14 is optical waveguide where the propagated light is recom